<div *ngIf="messageService.messages.length">
  <h2>Messages</h2>
    <button mat-raised-button 
            (click)="messageService.clear()">clear</button>
    <mat-list>
        <mat-list-item *ngFor='let message of messageService.messages'> {{message}} </mat-list-item>
    </mat-list>
  
</div>